HIGH STENGTH Al-Zn ALLOY AND METHOD FOR PRODUCING SUCH AN ALLOY PRODUCT

ABSTRACT

The present invention relates to a high strength Al—Zn alloy product with an improved combination of corrosion resistance and toughness, the alloy including essentially (in weight percent): Zn: 6.0-9.5, Cu: 1.3-2.4, Mg: 1.5-2.6, Mn and Zr&lt;0.25 but preferably in a range between 0.05 and 0.15 for higher Zn contents, other elements each less than 0.05 and less than 0.25 in total, balance aluminium, wherein (in weight percent): 0.1[Cu]+1.3&lt;[Mg]&lt;0.2[Cu]+2.15. The invention also relates to a method to produce these alloy products, and to some preferred applications thereof such as upper wing applications in aerospace.

CROSS-REFERENCE TO RELATED APPLICATIONS

This claims priority from U.S. provisional patent application Ser. No.60/463,723 filed Apr. 18, 2003 and European patent application No.03076049.0 filed Apr. 10, 2003, both incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The present invention relates to a wrought high strength Al—Zn alloyproduct with an improved combination of corrosion resistance andtoughness, a method for producing a wrought high strength Al—Zn alloyproduct with an improved combination of corrosion resistance andtoughness and a plate product of such alloy, optionally produced inaccordance with the method. More specifically, the present inventionrelates to a wrought high strength Al—Zn alloy designated by the7000-series of the international nomenclature of the AluminiumAssociation for structural aeronautical applications. Even morespecifically, the present invention relates to a new chemistry windowfor an Al—Zn alloy product having improved combinations of strength,toughness and corrosion resistance, which does not need specific agingor temper treatments.

BACKGROUND OF THE INVENTION

It is known in the art to use heat treatable aluminium alloys in anumber of applications involving relatively high strength, hightoughness and corrosion resistance such as aircraft fuselages, vehicularmembers and other applications. Aluminium alloys AA7050 and AA7150exhibit high strength in T6-type tempers, see e.g. U.S. Pat. No.6,315,842. Also precipitation-hardened AA7×75, AA7×55 alloy productsexhibit high strength values in the T6 temper. The T6 temper is known toenhance the strength of the alloy, wherein the aforementioned AA7×50,AA7×75 and AA7×55 alloy products which contain high amounts of zinc,copper and magnesium are known for their high strength-to-weight ratiosand, therefore, find application in particular in the aircraft industry.However, these applications result in exposure to a wide variety ofclimatic conditions necessitating careful control of working and agingconditions to provide adequate strength and resistance to corrosion,including both stress corrosion and exfoliation.

In order to enhance resistance against stress corrosion and exfoliationas well as fracture toughness it is known to artificially over-age theseAA7000-series alloys. When artificially aged to a T79, T76, T74 orT73-type temper their resistance to stress corrosion, exfoliationcorrosion and fracture toughness improve in the order stated (T73 beingbest and T79 being close to T6) but at cost to strength compared to theT6 temper condition. An acceptable temper condition is the T74-typetemper which is a limited over-aged condition, between T73 and T76, inorder to obtain an acceptable level of tensile strength, stresscorrosion resistance, exfoliation corrosion resistance and fracturetoughness. Such T74 temper is performed by over-aging the aluminiumalloy product at temperatures of 121° C. for 6 to 24 hours and 171° C.for about 14 hours.

Depending on the design criteria for a particular airplane componenteven small improvements in strength, toughness or corrosion resistanceresult in weight savings, which translate to fuel economy over the lifetime of the aircraft. To meet these demands several other 7000-seriestype alloys have been developed:

EP-0377779 discloses an improved process for producing a 7055 alloy forsheet or thin plate applications in the field of aerospace such asupper-wing members with high toughness and good corrosion propertieswhich comprises the steps of working a body having a compositionconsisting of, in wt. %:

-   -   Zn: 7.6-8.4    -   Cu: 2.2-2.6    -   Mg: 1.8-2.1,    -   one or more elements selected from    -   Zr: 0.5-0.2    -   Mn: 0.05-0.4    -   V: 0.03-0.2    -   Hf: 0.03-0.5,        the total of said elements not exceeding 0.6 wt. %, the balance        aluminium plus incidental impurities, solution heat treating and        quenching the product and artificially aging the product by        either heating the product three times in a row to one or more        temperatures from 79° C. to 163° C. or heating such product        first to one or more temperatures from 79° C. to 141° C. for two        hours or more or heating the product to one or more temperatures        from 148° C. to 174° C. These products show an improved        exfoliation corrosion resistance of “EB” or better with about        15% greater yield strength than similar sized AA7×50        counterparts in the T76-temper condition. They still have at        least about 5% greater strength than their similarly-sized        7×50-T77 counterpart (M7150-T77 will be used hereinbelow as a        reference alloy).

U.S. Pat. No. 5,312,498 discloses another method for producing analuminium-based alloy product having improved exfoliation resistance andfracture toughness with balanced zinc, copper and magnesium levels suchthat there is no excess of copper and magnesium. The method of producingthe aluminium-based alloy product utilizes either a one- or two-stepaging process in conjunction with the stochiometric balancing of copper,magnesium and zinc. A two-step aging sequence is disclosed wherein thealloy is first aged at approx. 121° C. for about 9 hours followed by asecond aging step at about 157° C. for about 10 to 16 hours followed byair-cooling. Such aging method is directed to thin plate or sheetproducts which are used for lower-wing skin applications or fuselageskin.

U.S. Pat. No. 4,954,188 discloses a method for providing a high strengthaluminium alloy characterised by improved resistance to exfoliationusing an alloy consisting of the following alloying elements, in wt. %:

-   -   Zn: 5.9-8.2    -   Cu: 1.5-3.0    -   Mg: 1.5-4.0    -   Cr: <0.04,        other elements such as zirconium, manganese, iron, silicon and        titanium in total less than 0.5, the balance aluminium, working        the alloy into a product of a pre-determined shape, solution        heat treating the reshaped product, quenching, and aging the        heat treated and quenched product to a temperature of from        132° C. to 140° C. for a period of from 6 to 30 hours. The        desired properties of having high strength, high toughness and        high corrosion resistance were achieved in this alloy by        lowering the aging temperature rather than raising the        temperature as taught previously from e.g. U.S. Pat. No.        3,881,966 or U.S. Pat. No. 3,794,531.

It has been reported that the known precipitation-hardened aluminiumalloys AA7075 and other AA7000-series alloys, in the T6 tempercondition, have not given sufficient resistance to corrosion undercertain conditions. The T7-type tempers which improve the resistance ofthe alloys to stress-corrosion cracking however decrease strengthsignificantly vis-a-vis the T6 condition.

U.S. Pat. No. 5,221,377 therefore discloses an alloy product consistingessentially of about 7.6 to 8.4 wt. % Zn, about 1.8 to 2.2 wt. % Mg andabout 2.0 to 2.6 wt. % Cu. Such alloy product exhibits a yield strengthwhich is about 10% greater than its 7×50-T6 counterpart with goodtoughness and corrosion resistance. The yield strength was reported tobe over 579 MPa with an exfoliation resistance (EXCO) level of “EC” orbetter.

U.S. Pat. No. 5,496,426 discloses an alloy as disclosed in U.S. Pat. No.5,221,377 and a process including hot rolling, annealing and coldrolling within a preferred cold reduction range of 20% to 70% which, inturn, is preferably followed by controlled annealing thereby displayingcharacteristics which are better than AA7075-T6 characteristics. Whilethe AA7075-T6 failed the stress corrosion resistance test (SCCresistance 40 days in the 35% NaCl alternate immersion test) at 138 MPathe disclosed processed alloy had a SCC resistance of 241 MPa.

U.S. Pat. No. 5,108,520 and U.S. Pat. No. 4,477,292 disclose an agingprocess for solution-heat-treated, precipitation hardening metal alloyincluding three steps of aging, comprising (1) aging the alloy at one ormore temperatures substantially above room temperature but below 163° C.to substantially below peak yield strength, (2) subsequently aging thealloy at one or more temperatures at about 190° C. for increasing theresistance of the alloy to corrosion and thereafter, (3) aging the alloyat one or more temperatures substantially above room temperature butbelow about 163° C. for increasing yield strength. The resultant productdisplayed good strength properties and a good corrosion performance.However, the three step aging procedure is cumbersome and difficult toperform so that the costs for producing such alloy increase.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide animproved Al—Zn alloy preferably for plate products with high strengthand an improved balance of toughness and corrosion performance. Morespecifically, it is the object of the present invention to provide analloy which can be used for upper wing applications in aerospace with animproved compression yield strength with properties which are betterthan the properties of a conventional AA7055-alloy in the T77 temper.

It is another object of the invention to obtain an AA7000-seriesaluminium alloy which exhibits strength in the range of T6-type tempersand toughness and corrosion resistance properties in the range ofT73-type tempers.

It is furthermore an object of the present invention to provide an alloythat can be used in an age-creep forming process, which is an alloywhich does not need a complicated or cumbersome aging process.

The present invention has a number of preferred objects.

As will be appreciated herein below, except otherwise indicated, alloydesignations and temper designations refer to the Aluminum Associationdesignations in Aluminum Standards and Data and the RegistrationRecords, all published by the US Aluminum Association. All percentagesare in weight percents, unless otherwise indicated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above mentioned objects of the invention are achieved by using ahigh strength Al—Zn alloy product with an improved combination ofcorrosion resistance and toughness, the alloy comprising essentially (inwt. %):

-   -   Zn about 6.0 to 9.5    -   Cu about 1.3 to 2.4    -   Mg about 1.5 to 2.6    -   Mn<0.12    -   Zr<0.20, and preferably 0.05-0.15    -   Cr<0.10    -   Fe<0.25, and preferably <0.12    -   Si<0.25, and preferably <0.12    -   Ti<0.10    -   Hf and/or V<0.25, and

optionally Ce and/or Sc<0.20, especially in a range of 0.05 to 0.15,other elements each less than 0.05 and less than 0.25 in total, balancealuminium, wherein (in weight percent):

0.1[Cu]+1.3<[Mg]<0.2[Cu]+2.15,

and preferably 0.2[Cu]+1.3<[Mg]<0.1[Cu]+2.15.

Such chemistry window for an AA7000-series alloy exhibits excellentproperties when produced to thin plate products which is preferablyusable in aerospace upper-wing applications. In the presentspecification all percentages are weight percents unless otherwiseindicated.

The above defined chemistry has properties which are comparable orbetter than existing alloys of the AA7×50 or AA7×55 series in theT77-temper, without using the above described cumbersome and complicatedT77 aging cycles. The chemistry leads to an aluminium product which isnot only superior with regard to the question of costs but also simplerto produce since less processing steps are necessary. Additionally, thechemistry allows new manufacturing techniques like age creep formingwhich is not feasible when a T77-temper alloy is applied. Even better,the chemistry as defined above can also be aged to the T77-temperwherein the corrosion resistance further improves as compared to thetwo-step aging procedure which is described hereinbelow, whereinespecially the exfoliation corrosion performance is enhanced.

By way of this invention it has been found that a selected range ofelements, using a higher amount of Zn and a specific combination of aparticular range of Mg and Cu, exhibit substantially better combinationsof strength, toughness and corrosion performance such as exfoliationcorrosion resistance and stress corrosion cracking resistance.

While it has been reported that copper contents should be maintainedhigher, preferably above about 2.2 wt. % in order to improve theexfoliation and stress corrosion cracking performance, bettercombinations of strength and density were reported to be achievable withrelatively low zinc contents.

In this invention, however, it has been found that elevated amounts ofzinc together with an optimised relation of magnesium to copper resultsin better strength while maintaining a good corrosion performance and atoughness which is better than conventional T77-temper alloys. It istherefore advantageous to have a combined zinc, magnesium and coppercontent in a range of between about 11.50 and 12.50 (in wt. %) withoutany manganese and below 11.00 in the presence of manganese which ispreferably between 0.06 and 0.12 (in wt. %).

A preferred amount of magnesium is in a range of0.2[Cu]+1.3<[Mg]<0.1[Cu]+2.15, most preferably in a range of0.2[Cu]+1.4<[Mg]<0.1[Cu]1.9. Copper is in a range of about 1.5 to 2.1,more preferably in a range of 1.5 to less than 2.0. The balance ofmagnesium and copper is important for the inventive chemistry.

Copper and magnesium are important elements for adding strength to thealloy. Too low amounts of magnesium and copper result in a decrease ofstrength while too high amounts of magnesium and copper result in alower corrosion performance and problems with the weldability of thealloy product. Prior art techniques used special aging procedures toameliorate the strength and low amounts of magnesium and copper are usedin order to achieve a good corrosion performance. In order to achieve acompromise in strength, toughness and corrosion performance copper andmagnesium amounts (in wt. %) of between about 1.5 and 2.3 have beenfound to give a good balance for thick alloy products. However, thecorrosion performance is the vital parameter for thin alloy products sothat less amounts of copper and magnesium must be used, therebyresulting in a lower strength. Throughout the claimed chemistry of thepresent invention it is now possible to achieve strength levels in theregion of a T6-temper alloy while maintaining corrosion performancecharacteristics similar to those of T74-temper alloys.

Apart from the amounts of magnesium and copper the invention discloses abalance of magnesium and copper amounts to zinc, especially the balanceof magnesium to zinc, which gives the alloy these performancecharacteristics. The improved corrosion resistance of the alloyaccording to the invention has exfoliation resistance properties(“EXCO”) of EB or better, preferably EA or better.

These exfoliation properties are measured in accordance with thestandards for resistance to stress corrosion cracking (“SCC”) andexfoliation resistance (“EXCO”) currently required for AA7075, AA7050and AA7150-products aged to the T73, T74 and T76, along with typicalperformance of T6, tempers. To determine whether commercial alloys meetthe SCC standards, a given test specimen is subjected to predefined testconditions. Bar-shaped specimens are exposed to cycles of immersing in a3.5% NaCl aqueous solution for 10 minutes, followed by 50 minutes of airdrying while being pulled from both ends under a constant strain (stresslevel). Such testing is usually carried out for a minimum of 20 days (orfor less time should the specimen fail or crack before 20 days havepassed). This test is the ASTM standard G47 (G47-98) test.

Another preferred SCC-test, conducted in accordance with ASTM standardG47, (G38-73) is used for extruded alloy products that include thinplate products. This test consists of compressing the opposite ends of aC-shaped ring using constant strain levels and alternate immersionconditions substantially similar to those as described above. While anAA7075, AA7050 or AA7150-T6 tempered alloy fails the SCC test in lessthan 20 days and while the exfoliation properties are EC or ED, thecorrosion resistance performance increases with tempers T76-, T74-, T73.The exfoliation properties of T73 are EA or better. Specific examplesare described hereinbelow.

The inventive alloy has a chemistry with a preferred amount of magnesiumand copper of about 1.93 when the amount (in wt. %) of zinc is about8.1. However, the amount (in wt. %) of zinc is in a range of 6.1 to 8.3,more preferably in a range of 6.1 to 7.0 if manganese is lower than0.05, and preferably lower than 0.02. Some preferred embodiments of thepresent invention are described within the examples hereinbelow.

The amount of manganese (in wt. %) is preferably in a range of about0.06 to 0.12 when the amount of zinc is above 7.6. Manganese contributesto or aids in grain size control during operations that can cause thealloy microstructure to recrystallize. The preferred levels of manganeseare lower than in conventional AA7000-series alloys but may be raisedwhen zinc is raised.

The amount of the additional alloying elements Ce and/or Sc is smallerthan 0.20, preferably in a range of 0.05 to 0.15, most preferably around0.10.

A preferred method for producing a wrought high strength Al—Zn alloyproduct with an improved combination of corrosion resistance andtoughness comprises the steps of

a) casting an ingot with the following composition (in weight percent):

-   -   Zn about 6.0 to 9.5    -   Cu about 1.3 to 2.4    -   Mg about 1.5 to 2.6    -   Mn<0.12    -   Zr<0.20, preferably 0.05-0.15    -   Cr<0.10    -   Fe<0.25    -   Si<0.25    -   Ti<0.10    -   Hf and/or V<0.25, optionally Ce and/or Sc<0.20,        other elements each less than 0.05 and less than 0.25 in total,        balance aluminium, and wherein (in weight percent):

0.1[Cu]+1.3<[Mg]<0.2[Cu]+2.15,

b) homogenising and/or pre-heating the ingot after casting,

c) hot working the ingot and optionally cold working into a workedproduct,

d) solution heat treating at a temperature and time sufficient to placeinto solid solution essentially all soluble constituents in the alloy,and

e) quenching the solution heat treated product by one of spray quenchingor immersion quenching in water or other quenching media.

The properties of the invention may be further achieved throughout apreferred method which includes artificially aging the worked andsolution heat-treated product, wherein the aging step comprises a firstheat treatment at a temperature in a range of 105° C. to 135° C.,preferably around 120° C. for 2 to 20 hours, preferably around 8 hours,and a second heat treatment at a higher temperature than 135° C. butbelow 210° C., preferably around 155° C. for 4 to 12 hours, preferably 8to 10 hours.

Throughout such two-step aging treatment a corrosion performance isachieved which is similar to the corrosion performance of a T76-temperalloy. However, it is also possible to artificially aging the worked andheat treated product wherein the aging step comprises a third heattreatment at a temperature in a range of 105° C. to 135° C. for morethan 20 hours and less than 30 hours. This T77-temper aging procedure isknown and even increases the performance characteristics as compared tothe two-step aging procedure. However, the two-step aging procedureresults in thin aluminium alloy products which are partially comparableand partially better than T77-temper products.

It is furthermore possible to artificially aging the worked and heattreated product with a two-step aging procedure to a T79- or T76-temper.After homogenizing and/or pre-heating the ingot after casting it ispreferably advisable to hot working the ingot and optionally coldworking the hot worked products into a worked product of 15 mm to 45 mm,thereby obtaining a thin plate.

Such plate product of high strength Al—Zn alloy may be obtained by analloy having a composition as described above or being produced inaccordance with a method as described above. Such plate product ispreferably usable as thin aircraft member, more preferably as anelongated structural shape member. Even more preferred is a plateproduct for use as an upper-wing member, preferably a thin skin memberof an upper-wing or of a stringer of an aircraft.

The foregoing and other features and advantages of the alloys accordingto the invention will become readily apparent from the followingdetailed description of preferred embodiments.

Example 1

Tests were performed comparing the performance of the alloy according tothe present invention and AA7150-T77 alloys. It has been found that theexamples of the alloy of the present invention show an improvement overconventional M7150-T77-temper alloys.

On an industrial scale four different aluminium alloys have been castinto ingots, homogenized, preheated for more than 6 hours at 410° C. andhot rolled to 30 mm plates. Thereafter, the plates were solution heattreated at 475° C. and water quenched. Thereafter, the quenched productwas aged by a two-step T79-T76 aging procedure. The chemicalcompositions are set out in Table 1.

TABLE 1 Chemical composition of thin plate alloys, in wt. %, balancealuminium and inevitable impurities, Alloys 1 to 4 with Mn ≦ 0.02: Si FeCu Mn Mg Cr Zn Ti Zr Alloy 1 0.03 0.06 2.23 0.00 2.08 0.00 6.24 0.030.10 (7050) Alloy 2 0.05 0.08 2.05 0.01 2.04 0.01 6.18 0.04 0.11 Alloy 30.05 0.09 2.20 0.01 2.30 0.01 7.03 0.04 0.10 Alloy 4 0.04 0.07 1.91 0.022.13 0.00 6.94 0.03 0.11

The aged alloys were thereafter tested in accordance with the followingtest conditions:

Tensile yield strength was measured according to EN 10.002, exfoliationresistance properties (“EXCO”) were measured according to ASTM G-34-97,stress corrosion cracking (“SCC”) was measured according to ASTMG-47-98, all in ST-direction, Kahn-Tear (toughness) was measuredaccording to ASTM E-399 and the compression yield strength (“CYS”) wasmeasured according to ASTM E-9.

The results of the T79-T76 aged plate products of the four alloys asshown in Table 1 are shown in Table 2a when compared with conventionalAA7150-T77 tempered alloys, and in Table 2b when compared withconventional M7150-T76/T74/T6 tempered alloys:

TABLE 2a Overview of strength and toughness of the alloys of Table 1 (30mm plates) compared to three reference alloys (AA7150- T77); alloys 1 to4 aged to T79-T76: Rp-L CYS-LT K_(1C)-LT (MPa) (MPa) EXCO (MPa√m) Alloy1 555 565 EC 35.1 Alloy 2 561 604 EA/B 34.5 Alloy 3 565 590 EB 29.1Alloy 4 591 632 EB 28.9 AA7150-T77 586 — EB 28.6 AA7150-T77 579 — EB29.2 AA7150-T77 537 — EA 33.2 F = no failure after 40 days.

TABLE 2b Overview of corrosion performance of the alloys of Table 1 (30mm plates) compared to three reference alloys (AA7150- T76, AA7150-T74,AA7150-T6); alloys 1-4 aged to T79-T76: SCC threshold Alloy 1 NF at 172MPa Alloy 2 NF at 240 MPa Alloy 3 NF at 240 MPa Alloy 4 NF at 240 MPaAA7150-T76 117-172 MPa AA7150-T74 240 MPa AA7150-T6 <48 MPa NF = nofailure after 40 days.

As can be seen from Tables 2a, 2b alloys 1, 2 and 4 show betterstrength/toughness combinations. Alloys 2, 3 and 4 all have anacceptable EXCO performance wherein alloys 2, 3 and 4 have a significanthigher compression yield strength than alloy No. 1 (AA7050-alloy).Alloys 2 and 4 exhibit a property balance that makes them very suitablefor upper-wing applications in aerospace thereby showing a balance ofproperties which is better than those of conventional 7150-T77 alloys.However, it is still possible to use a T77-temper for the inventivealloys as shown in Table 3.

TABLE 3 Alloys 2 and 4 tempered according to T77 temper conditions,overview of strength, toughness and corrosion performance. Rp-L CYS-LTK_(1C)-LT (MPa) (MPa) EXCO (MPa√m) SCC threshold Alloy 2 585 613 EA 32.2NF at 240 MPa Alloy 4 607 641 EA 26.4 NF at 240 MPa

Further SCC testing was performed on the promising alloy No. 4 whereinalloy 4-samples were prepared according to the procedure described inASTM G-47-98 (standard test methods for determining susceptibility tostress corrosion cracking of AA7000-series aluminium alloy products) andexposed to the corrosive atmosphere according to ASTM G-44-94 (alternateimmersion in accordance with the standard practice for evaluating stresscorrosion cracking resistance of metals and alloys by alternateimmersion in 3.5% NaCl solution).

Four different stress levels were chosen for samples of alloy 4 as shownin Table 4. For each stress level three samples were exposed to the testenvironment (ASTM G-44). One was taken out after 1 week while the othertwo were exposed for 40 days. When no cracking had occurred during theexposure the tensile properties were determined as shown in Table 4.

TABLE 4 Overview of tensile strength properties of alloy 4 afterexposure to four different stress levels, pre-stress was acting in LTdirection. Tensile strength Pre-stress [MPa] Alloy 4 [MPa] 1 week 40days 300 524.3 428.0 340 513.1 416.9 380 503.1 424.5 420 515.5 425.1

As can be seen from Table 4, no decrease in residual strength wasmeasured with increasing load which means that no measurable stresscorrosion appeared after 40 days as far as tensile strength propertiesare concerned.

Example 2

When higher strength levels are required and toughness properties areless important conventional AA7055-T77 alloys are preferred instead ofalloys as an alloy for upper wing applications. The present inventiontherefore discloses optimised copper and magnesium windows which showproperties equal or better than conventional AA7055-T77 alloys.

11 different aluminium alloys were cast into ingots having the followingchemical composition as set out in Table 5.

TABLE 5 Chemical composition of 11 alloys, in wt. %, balance aluminiumand inevitable impurities, Zr = 0.08, Si = 0.05, Fe = 0.08. Alloy Cu MgZn Mn 1 2.40 2.20 8.2 0.00 2 1.94 2.33 8.2 0.00 3 1.26 2.32 8.1 0.00 42.36 1.94 8.1 0.00 5 1.94 1.92 8.1 0.00 6 1.30 2.09 8.2 0.00 7 1.92 1.548.1 0.00 8 1.27 1.57 8.1 0.00 9 2.34 2.25 8.1 0.07 10 2.38 2.09 8.1 0.0011 2.35 1.53 8.2 0.00

Strength and toughness properties were measured after pre-heating thecast alloys for 6 hours at 410° C. and then hot rolling the alloys to agauge of 28 mm. Thereafter, solution heat treating was applied at 475°C. and water quenching. Aging was done for 8 hours at 120° C. and 8 to10 hours at 155° C. (T79-T76-temper). The results are shown in Table 6.

TABLE 6 Overview of strength and toughness of 11 alloys according toTable 5 in the identified directions. Rp Rm Kq Alloy L LT L LT L-T 1 628596 651 633 28.9 2 614 561 642 604 29.3 3 566 544 596 582 39.0 4 614 568638 604 33.0 5 595 556 620 590 37.1 6 562 513 590 552 38.6 7 549 509 573542 41.7 8 530 484 556 522 41.9 9 628 584 644 618 26.6 10 614 575 631606 28.1 11 568 529 594 568 36.6

While alloys 3 to 8 and 11 displayed good toughness properties alloys 1to 5 and 9 and 10 displayed good strength properties. Hence, alloys 3, 4and 5 show a good balance of strength and toughness so that it is clearto have a copper content of above 1.3 and a magnesium content of above1.6 (in wt. %) when zinc is present in an amount of 8.1. Such amountsare lower limits for the copper and magnesium windows. As can be seenfrom Table 6 the toughness will drop to un-acceptable low-levels whencopper and magnesium levels are too high (alloys 1, 2, 9 and 10).

Example 3

The influence of manganese was investigated on the properties of theinventive alloy. An optimum manganese level was found between 0.05 and0.12 in alloys with a high amount of zinc. The results are shown inTables 7 and 8. All not mentioned chemistry properties and processingparameters are similar to those of Example 2.

TABLE 7 Chemical composition of three alloys (Mn-0, Mn-1 and Mn-2), inwt. %, balance aluminium and inevitable impurities, Zr = 0.08, Si =0.05, Fe = 0.08. Alloy Cu Mg Zn Mn Mn-0 1.94 2.33 8.2 0.00 Mn-1 1.942.27 8.1 0.06 Mn-2 1.96 2.29 8.2 0.12

TABLE 8 Overview of strength and toughness of three alloys according toTable 7 in the identified directions. Rp Rm Kq Alloy L LT L LT L − TMn-0 614 561 642 604 29.3 Mn-1 612 562 635 602 31.9 Mn-2 612 560 639 59629.9

As shown in Table 8 the toughness properties decrease while strengthproperties increase. For alloys with high amounts of zinc an optimisedmanganese level is between 0.05 and 0.12.

Example 4

When higher strength levels are required and toughness properties areless important conventional AA7055-T77 alloys are preferred instead ofalloys as an alloy for upper wing applications. The present inventiontherefore discloses optimised copper and magnesium windows which showproperties equal or better to conventional M7055-T77 alloys.

Two different aluminium alloys were cast into ingots having thefollowing chemical composition as set out in Table 9.

TABLE 9 Chemical composition of three alloys, in wt. %, balancealuminium and inevitable impurities, Zr = 0.08, Si = 0.05, Fe = 0.08;(Ref = AA7055 alloy). Alloy Si Fe Cu Mn Mg Cr Zn Ti Zr 1 0.05 0.09 2.240.01 2.37 0.01 7.89 0.04 0.10 2 0.04 0.07 1.82 0.08 2.18 0.00 8.04 0.030.10 Ref. 2.1-2.6 1.8-2.2 7.6-8.4

Alloys 1 and 2 were tested with regard to their strength properties.These properties are shown in Table 10. Alloy 2 has been tempered inaccordance with two temper conditions (T79-T76 and T77). Reference alloyAA7055 has been measured in T77 temper (M-Ref) while the technical dataof an M7055 reference alloy in a T77 temper are given as well (asidentified by Ref).

TABLE 10 Overview of strength of the two inventive alloys of Table 9,alloy No. 2 in two temper conditions, reference alloy (AA7055) measured(M-Ref) and tech sheet (Ref). Alloy Temper Rp-L Rp-LT Rp-ST Rm-L Rm-LTRm-ST 1 T79-T76 604 593 559 634 631 613 2 T79-T76 612 598 571 645 634618 2 T77 619 606 569 640 631 610 Ref T77 614 614 — 634 641 — M-Ref T77621 611 537 638 634 599

The toughness properties in LT and TL direction as well as thecompression yield strength properties in L and LT direction as well asthe corrosion performance characteristics are shown in Table 11.

TABLE 11 Toughness and CYS properties of the two inventive alloys ofTable 9 in different temper conditions and different test directions, NF= no failure after 40 days at designated stress levels, otherwise daysare stated after which the specimen failed. K_(IC) K_(IC) CYS- AlloyTemper (L-T) (T-L) CYS-L LT EXCO SCC 1 T79-T76 21.0 — 596 621 EC 2, 3, 82 T79-T76 28.9 27.1 630 660 EB NF at 172 MPa 2 T77 28.8 26.5 628 656 EANF at 210 MPa Ref T77 28.6 26.4 621 648 EB NF at 103 MPa M-Ref T77 — — —— EB NF at 103 MPa

The inventive alloy has similar tensile properties as a conventionalAA7055-T77 alloy. However, the properties in the ST direction are betterthan those of the conventional M7055-T77 alloy. Also the stresscorrosion performance is better than of an AA055-T77 alloy. Theinventive alloy can therefore be used as an inexpensive substitute forAA7055-T77 tempered alloys which is also usable for age-creep forming,thereby showing a superior compression yield strength and corrosionresistance.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade without departing from the spirit or scope of the invention asherein described. The present invention is defined by the claimsappended hereto.

1. (canceled)
 2. The method according to claim 25, wherein the amount(in weight %) of Mg is in a range of 0.2[Cu]+1.3<[Mg]<0.1[Cu]+2.15. 3.The method according to claim 25, wherein the amount (in weight %) of Mgis in a range of 0.2[Cu]+1.4<[Mg]<0.1[Cu]+1.9.
 4. The method accordingto claim 25, wherein the alloy product has an exfoliation corrosionresistance (“EXCO”) of EB or better.
 5. The method according to claim25, wherein the alloy product has an exfoliation corrosion resistance(“EXCO”) of EA or better.
 6. The method according to claim 25, whereinthe amount (in weight %) of Cu is in a range of 1.5 to 2.1.
 7. Themethod according to claim 25, wherein the amount (in weight %) of Cu isin a range of 1.5 to 2.0.
 8. The method according to claim 25, whereinthe amount (in weight %) of Zr is in a range of 0.05 to 0.15.
 9. Themethod according to claim 25, wherein the amount (in weight %) of Mg andCu is about 1.93 when the amount (in weight %) of Zn is about 8.1. 10.The method according to claim 25, wherein the amount (in weight %) of Znis in a range of 6.1 to 8.3.
 11. The method according to claim 25,wherein the amount (in weight %) of Zn is in a range of 6.1 to 8.3 andMn is lower than 0.05.
 12. The method according to claim 25, wherein theamount (in weight %) of Mn is in a range of 0.06 to 0.12 when the amountof Zn is above 7.6.
 13. The method according to claim 25, wherein theamount (in weight %) of Fe is less than 0.12.
 14. The method accordingto claim 25, wherein the amount (in weight %) of Si is less than 0.12.15-16. (canceled)
 17. The method according to claim 25, wherein theproduct is a plate product.
 18. The method according to claim 25,wherein the product is a plate product having a thickness is a range of15 to 45 mm. 19-24. (canceled)
 25. A method for producing a wroughthigh-strength Al—Zn alloy product with an improved combination ofcorrosion resistance and toughness, comprising the steps of: a) castingan ingot with the following composition (in weight percent): Zn 6.0 to9.5 Cu 1.3 to 2.4 Mg 1.5 to 2.6 Mn<0.12 Zr<0.20 Cr<0.10 Fe<0.25 Si<0.25Ti<0.10 Hf and/or V<0.25, optionally Ce and/or Sc<0.20, other elementseach less than 0.05 and less than 0.50 in total, balance aluminium,wherein (in weight percent):0.1[Cu]+1.3<[Mg]<0.2[Cu]+2.15, b) homogenising and/or pre-heating theingot after casting, c) hot working the ingot and optionally coldworking into a worked product, d) solution heat treating and e)quenching the solution heat treated product.
 26. The method according toclaim 25 wherein the worked and solution heat-treated product isartificially aged, and wherein the aging step comprises a first heattreatment at a temperature in a range of 105° C. to 135° C. for 2 to 20hours, and a second heat treatment at a higher temperature than 135° C.but below 210° C. for 4 to 12 hours.
 27. The method according to inclaim 26, wherein the worked and solution heat-treated product isartificially aged, and wherein the aging step comprises a third heattreatment at a temperature in a range of 105° C. to 135° C. for morethan 20 hours and less than 30 hours.
 28. The method according to claim25, wherein the worked and solution heat-treated product is artificiallyaged, and wherein the aging step consists of a first heat treatment at atemperature in a range of 105° C. to 135° C. for 2 to 20 hours, and asecond heat treatment at a higher temperature than 135° C. but below210° C. for 4 to 12 hours.
 29. The method according to claim 25,characterized by artificially aging the worked and solution heat-treatedproduct with a two-step aging procedure to a T79 or T76 temper.
 30. Themethod according to claim 25, wherein after homogenising and/orpre-heating the ingot after casting, hot working the ingot andoptionally cold working into a worked product having a thickness in therange of 15 mm to 45 mm.
 31. The method according to claim 30, whereinthe worked product is a plate product and the plate product is a thinaircraft member.
 32. The method according to claim 30, wherein theworked product is a plate product and the plate product is an elongatestructural shape member of an aircraft.
 33. The method according toclaim 30, wherein the worked product is a plate product and the plateproduct is an upper-wing member of an aircraft.
 34. The method accordingto claim 30, wherein the worked product is a plate product and the plateproduct is a thin skin member of an upper-wing of an aircraft.
 35. Themethod according to claim 30, wherein the worked product is a plateproduct and the plate product is stringer of an aircraft.
 36. The methodaccording to claim 30, wherein the worked product is a plate product andthe plate product is stringer of an upper-wing of an aircraft.
 37. Themethod according to claim 25, wherein the amount (in weight %) of Zn isin a range of 6.1 to 8.3 and Mn is lower than 0.02.
 38. The methodaccording to claim 25, wherein the amount (in weight %) of Zn is in arange of 6.1 to 7.0 and Mn is lower than 0.05.
 39. The method accordingto claim 25, wherein the amount (in weight %) of Zn is in a range of 6.1to 7.0 and Mn is lower than 0.02.